Molecular imaging has the potential to detect disease progression or therapeutic effectiveness earlier than most conventional methods in the fields of oncology, neurology and cardiology. Of the several promising molecular imaging technologies having been developed as optical imaging and MRI. PET is of particular interest for drug development because of its high sensitivity and ability to provide quantitative and kinetic data.
For example, positron emitting isotopes include carbon, iodine, fluorine, nitrogen, and oxygen. These isotopes can replace their non-radioactive counterparts in target compounds to produce tracers that function biologically and are chemically identical to the original molecules for PET imaging, or can be attached to said counterparts to give close analogues of the respective parent effector molecule. Among these isotopes 18F is the most convenient labeling isotope due to its relatively long half life (110 min) which permits the preparation of diagnostic tracers and subsequent study of biochemical processes. In addition, its high β+ yield and low β+ energy (635 keV) are also advantageous.
The 18F-fluorination reaction is of great importance for 18F-labeled radiopharmaceuticals which are used as in vivo imaging agents targeting and visualizing diseases, e.g. solid tumours or diseases of brain. A very important technical goal in using 18F-labeled radiopharmaceuticals is the rapid preparation and administration of the radioactive compound due to the fact that the 18F isotopes have a half-life of about 110 minutes that is beneficial for clinical use on the one hand, but is challenging for production processes of such compounds on the other hand.
The best known example for PET imaging of diseases is 2-[18F]fluorodeoxyglucose ([18F]FDG), which is the most widely used PET radiopharmaceutical [J Nucl Med (1978), 19: 1154-1161].
However, a number of pitfalls and artifacts have been ascribed to FDG imaging and more continue to surface as the worldwide experience with FDG increases. Altered FDG uptake in muscles, brown adipose tissue, bone marrow, the urinary tract, and the bowel is demonstrated in a significant proportion of patients, which can hide underlying malignant foci or mimic malignant lesions (Seminars in Nuclear Medicine, 40, 283 (2010)). Although PET is a sensitive tool for detecting malignancy. FDG uptake is not tumor specific. It can also be seen in healthy tissue or in benign disease as inflammation or posttraumatic repair and could be mistaken for cancer. The experienced nuclear medicine physician mostly manages to differentiate malignant from non-malignant FDG uptake, but some findings may remain ambiguous (Euro. Radio., 16, 1054 (2006)).
The area most common for interpretative pitfalls with FDG is related to uptake in active skeletal muscle. Many benign conditions can cause high accumulation of FDG creating the potential for false positive interpretation. Such pitfalls include variable physiologic FDG uptake in the digestive tract, thyroid gland, skeletal muscle, myocardium, bone marrow, and genitourinary tract and benign pathologic FDG uptake in healing bone, lymph nodes, joints, sites of infection, and cases of regional response to infection and aseptic inflammatory response. In many instances, these physiologic variants and benign pathologic causes of FDG uptake can be specifically recognized and properly categorized; in other instances, such as the lymph node response to inflammation or infection, focal FDG uptake is non-specific (J. Nucl. Med. Tech. 33, 145 (2005), Radiographics, 19, 61 (1999), Seminars in Nuclear Medicine, 34, 122 (2004); 34, 56 (2004), J. Nucl. Med. 45, 695 (2004)).
To overcome at least some of these limitations of FDG, other adaptations of the tumor metabolism beyond enhanced glycolysis need to be exploited to provide an improved PET imaging agent for tumors. Tumors in general often have to cope with severe conditions of oxidative stress. Thiol containing molecules like the amino acid L-cysteine and the tripeptide glutathione (GSH) are the major cellular components to overcome these conditions and are consumed for detoxification of reactive oxygen species (ROS) and other electrophiles, such as chemotherapeutics. Thus, a continuous supply of GSH and its precursors are critical for cell survival and provide a selective advantage for tumor growth. L-Cysteine plays a key role as reactive oxygen species scavenger by itself and is the rate-limiting building block for GSH biosynthesis. In the blood the oxidized dimer L-cystine is the dominant form and the availability of L-cysteine is limited. However, L-cystine can be efficiently provided to cells via the cystine/glutamate exchanger xCT (SLC7A11). Subsequently L-cystine is reduced inside the cells to yield two molecules of L-cysteine. An increased xCT expression is found in many tumors. The xCT transporter was first described by Bannai and Kitamura in 1980 as a Na+-independent, high-affinity transporter for L-cystine and L-glutamate in human fibroblasts (J. Biol. Chem. 255 (1980) 2372-2376). It is a heteromeric amino acid transporter for anionic amino acids and the main transporter for L-cystine (Pflugers Arch 442 (2001) 286-296, Pflugers Arch 447 (2004) 532-542). It is important to note that the xCT transporter is not able to discriminate between its natural substrates L-cystine and L-glutamate for the inward directed transport (Neuropharmacology 46 (2004) 273-284). Radiolabeled amino acid derivatives targeting the xCT transporter have been described before. [18F]fluoroalkyl- and [18F]fluoroalkoxyl-substituted glutamic acid derivatives are disclosed in WO2008052788, WO2009141091, WO2009141090. Furthermore, WO2009141090 comprises fluorobenzyl- and fluoropyridylmethyl-substituted glutamic acid derivatives.
As one example, 4-(3-[18F]fluoropropyl)glutamic acid was shown to be a promising agent for tumor imaging (WO2009141091, example 4). A biodistribution study in A549 tumor bearing nude mice demonstrated very good tumor targeting (1.6% ID/g at 1 h post injection), Kidney (6.4% ID/g at 30 min post injection) and pancreas (8.5% ID/g at 30 min post injection) were found to be the non-tumor organs with highest tracer uptake. Beside 18F labeled glutamic acid derivatives, also 18F labeled cystine derivatives were investigated regarding the potential to target xCT activity in tumors (WO2010125068). However, it is well known, that the uptake of L-cystine is pH dependent, with less uptake at low pH as typically found in tumor environment. In contrast, uptake of glutamate and its derivatives via xCT is independent of pH (J Biol Chem 256 (1981) 5770-5772).
Due to the half life of 18F isotope of about 110 min, an 18F radiopharmaceutical demands a daily production. Key factor for successful routine use of such radiotracer are a robust and high yield radiosynthesis on the one hand and reliable and fast analytical methods for determination of radiochemical purity, specific activity and by-products. Well established analytical methods comprise liquid chromatography (e.g. HPLC, UPLC) using radiodetector and UV-detector.
A drawback of the most promising 18F labeled xCt substrates described so for is a missing chromophor to enable a standard detection by UV detectors. To measure specific activity or presence of by-products, derivatization methods (e.g. pre-column derivatization, post-column derivatization) using for example OPA, ACCQ, Fmoc, Ninhydrin reagents can be used. However, a simple direct analysis of the radiopharmaceutical compositions would be advantageous.